This invention relates to temperature control techniques and more specifically to a technique for controlling the temperature within a serviced area or zone by variably adjusting the run time of a fan during successive fan cycles.
The control of temperature within a prescribed or serviced area, such as a work area, an office area, a school room, or an area of the home, has long been a desired goal of the heating and air conditioning industry. In particular, it is desired to achieve this goal in a manner which is cost efficient, and yet maintains the temperature within comfortable limits for an extended period of time.
The most primitive type of temperature control within a serviced area is an ON-OFF switch. When the occupant is too warm or too cool, the switch is turned on to provide cool or heated air, whichever is desired. The next step in the development of the art is the use of a thermostat, which in actuality, senses the temperature in the serviced area and automatically turns the aforementioned switch on or off, which ever is needed. While this approach achieves a desired temperature level, it is not very cost efficient, because whenever a temperature varies slightly, the fan and heat exchanger or cooling coils are activated until the target temperature is met.
A further approach is to operate the fan continuously, but vary the temperature of the heat exchanger. Varying the temperature of the heat exchanger is accomplished by cycling the run time of the compressor or varying the volume of the heating or cooling medium. Such is an expensive undertaking and difficult to achieve consistently.
Another technique for controlling the temperature in a serviced area is by throttling the air volume (as by dampers) delivered to the serviced zone or throttling the air volume delivered to the main fan. These techniques generally result in an undesirable situation because the balance within the overall HVAC system is disrupted, and thus it is difficult to achieve proper control.
In the present invention, on the other hand, control of the temperature in the serviced zone is maintained within desired limits by establishing computer controlled time periods or cycles in which the fan is operative part of the time and inoperative part of the time. The relative time in each period during which the fan runs may vary depending on the measured conditions in the previous cycle. The amount of time the fan is operated per cycle is determined by measuring the temperature during the previous cycle. The difference between the average measured temperature and a prescribed "balance temperature" determines how long the fan operates during the next cycle. The "balance temperature" is a prescribed temperature toward which the system is aiming to achieve a comfortable continuous environment. The measuring and adjusting is repeated each cycle, so that the fan run time for each cycle is determined and the temperature is actually modulated toward the prescribed limit.
The primary advantage of this approach is to maximize the need at a minimum cost. As additional capacity is needed (for example as when it gets hotter outside) the run time of the fan increases. Conversely, as the capacity needs are diminished the run time decreases.
Another advantage of the present invention lies in the fact that it is possible to maintain a constant volume of air flow through the duct system while maintaining an air distribution balance. Further, in systems which require some ventilation, whether or not heating or cooling is needed, the invention guarantees a minimum run time of the fan every cycle, even while the temperature is remaining close to the prescribed limit.
More particularly, the technique according to the present invention accomplishes a control of the temperature in the served zone by operating the fan in cycles of predetermined time span, wherein the length of time in each cycle in which fan is operated is variably controlled responsive to fluctuations in the difference between the actual temperature and the zone being serviced and a "balance temperature." In effecting this result, the technique envisions setting prescribed cycle periods of a time duration during which the fan is operative and inoperative. A prescribed balance temperature for the zone to be served by the equipment is determined. During each cycle, the temperature in the zone is continuously or intermittently measured. The average measured temperature is compared with the balance temperature being worked toward, then the fan run time for the next cycle is calculated according to a prescribed formula responsive to the measured difference. For example, during summertime operation the fan may be set to run a minimum of six out of every fifteen minutes. If during a fifteen minute cycle when the temperature measurement is made, and it is determined that the average temperature is too high (moving away from the balance temperature), then the fan run time for the next cycle is increased. During the next cycle the measurement is again made and adjusted responsive to the difference between the actual average temperature and the balance temperature being worked toward. In wintertime operation, the procedure works exactly oppositely.
It is therefore an object of the present invention to provide a temperature control technique which maximizes the need at a minimum cost.
It is another object of the present invention to provide a temperature control technique of the type described in which the main fan run time for successive cycles varies responsive to the difference between the actual average temperature in the zone to be treated and a balance temperature during the previous cycle.